1. Field of the Invention
The present invention relates to a structure for a plunger pump which can be optimally applied for an anti-skid brake control system for an automotive vehicle, and specifically to a support structure for a plunger operably employed in a plunger pump.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various automotive brake control systems associated with at least one of advantageous braking force control systems, namely a traction control system generally abbreviated as a TCS which is provided for suppressing excessive driving force exerted on driven wheels during quick depressing operation of an automotive accelerator pedal, such as quick starting, quick acceleration, or the like, and an anti-skid brake control system generally abbreviated as an ABS which is provided for preventing brakes from locking vehicle wheels during quick braking or during braking on a low frictional road surface so as to provide maximum effective braking. Of these braking force control systems, the ABS generally includes at least one brake fluid reservoir associated with either the front-wheel side or the rear-wheel side for temporarily storing the brake fluid returned from the wheel cylinders, and an auxiliary pump unit arranged for supplying the brake fluid in the wheel cylinders to the brake fluid reservoir so as to reduce the wheel cylinder pressure during the anti-skid brake control. The ABS also includes a pressure intensifying valve and a pressure reducing valve both connected to the associated wheel cylinder. The pressure intensifying valve is provided between the outlet port of the master cylinder and the inlet port of the wheel cylinder so as to establish or block the brake fluid flow from the master cylinder to the wheel cylinder, while the pressure reducing valve is provided between the brake fluid reservoir and the wheel cylinder so as to return the brake fluid in the wheel cylinder through the brake fluid reservoir and the ABS pump to the master cylinder. The ABS is operated so that the wheel slippage can be maintained within a predetermined slippage criterion by reducing, holding, and increasing the wheel cylinder pressure in response to the road condition. Traditionally, the pressure intensifying valve and the pressure reducing valve are each comprised of a two-position electromagnetic solenoid valve. Alternatively, the pressure intensifying valve and the pressure reducing valve are both comprised of a single three-position solenoid valve. For instance, the pressure intensifying valve is closed and the pressure reducing valve is opened in order to control the wheel slippage within an acceptable slippage, when the wheel cylinder pressure is reduced during the anti-skid brake control. The auxiliary pump unit is usually referred to as an "ABS pump". One such ABS pump has been disclosed in Japanese Patent First Publication Tokkai Heisei No. 2-286883.
As shown in FIG. 4, the above-noted conventional ABS pump unit is comprised of a plunger pump. Referring now to FIG. 4, the plunger pump comprises a pump housing 1 defining a cylindrical hollow 2 therein, a cylinder 3 inserted and fixed into the cylindrical hollow 2, and a plunger 4 slidably enclosed in the cylinder 3. The pump housing 1 is integrally formed as the same housing as the pressure intensifying and reducing valve being comprised of a three-position solenoid valve, for example. As clearly seen in FIG. 4, the cylinder 3 has a radial opening 3a, while the plunger 4 is formed with a radial opening 4a and an axial bore 4b. The plunger pump includes a plunger biasing means, such as a coil spring 7 comprised of a compression spring, for normally biasing the plunger 4 to a drive cam 8. The plunger pump also includes a check valve means for regulating the outlet pressure of the plunger pump. The check valve means is comprised of an inlet check ball 6 and a coil spring 5 whose spring constant determines a setting pressure of the check valve means. Conventionally, the drive cam 8 is comprised of an eccentric cam having a driven connection with a direct current motor. The plunger pump is driven by virtue of the DC motor. During operation of the plunger pump, the brake fluid temporarily stored in the brake fluid reservoir is introduced through the radial openings 3a and 4a into the axial bore 4b. Thereafter, the brake fluid in the axial bore 4b is fed through the inlet check valve 6 and 5 to an outlet port of the plunger pump, in accordance with a reciprocating motion of the plunger 4.
As set forth above, since the plunger 4 of the prior art plunger pump is normally biased in a direction outwardly projecting from the interior of the cylinder 3 by means of the coil springs 7 and 5, when a sub-assembly being in a state wherein the plunger 4, the return spring 7 for the plunger, the inlet check ball 6, and the return spring 5 for the check ball 6 are all assembled in the cylinder 3, is inserted into the cylindrical hollow 2 of the housing, there is a tendency for the plunger 4 and the check ball 6 to fall out of the cylinder 3. Therefore, when assembling the sub-assembly with the housing, a working efficiency is lowered and consequently a manufacturing cost is increased. Due to the outwardly projecting plunger of the sub-assembly, a degree of freedom for assembling is limited. That is, although the sub-assembly can be upwardly assembled into the cylindrical hollow 2 of the housing 1 in its vertical direction, it is difficult to downwardly insert the sub-assembly into the cylindrical hollow 2 of the housing in a vertical direction or to horizontally insert the sub-assembly into the cylindrical hollow 2, because the plunger and the inlet check valve both tends to fall out of the cylindrical hollow of the pump housing.